1. Technical Field
The present invention relates to a structure of a semiconductor device including an active region on a hetero junction of a nitride semiconductor.
2. Description of the Related Art
As a semiconductor device using a compound semiconductor, specifically, a device for high power/high frequency, a High Electron Mobility Transistor (HEMT) device using gallium nitride (GaN) has been employed. A schematic structure of a cross-section of the HEMT device disclosed in JP-A-2008-539587 is shown in FIG. 5. In this FIG. 5, an electron transit layer (channel layer) 93 and an electron supply layer (barrier layer) 94 are formed on a substrate 91 by epitaxial growth method. Here, the substrate 91 is made of aluminum nitride (AlN), the electron transit layer 93 is made of non-doped GaN, and the electron supply layer 94 is made of non-doped AlN. The electron transit layer 93 makes a hetero junction with the substrate 91, and the electron supply layer 94 makes a hetero junction with the electron transit layer 93. Meantime, the term “non-doped” means that an impurity injection is not performed in order to control a conductance level.
In the HEMT device 90, a two-dimensional electron gas layer (2DEG layer) 98 is formed within the electron transit layer 93 based on the hetero junction between the electron transit layer 93 and the electron supply layer 94. The 2DEG layer 98 is a current path flowing electric current between a source electrode 95 and a drain electrode 96. Here, a gate electrode 97 turns on/off the 2DEG layer 98 by voltage applied to the gate electrode 97 and performs the switching operation of the HEMT device 90.
At this time, because speed (mobility rate) of the electrons within the 2DEG layer 98 is extremely high, the HEMT device 90 will be operated at high speed. Moreover, since GaN has larger band-gap energy than gallium arsenide (GaAs), etc., the HEMT device 90 has high insulation withstand voltage and may be perform high power operation. Moreover, because AlN forming the substrate 91 has high insulation property, leak current flowing in a vertical direction of the HEMT device 90 can be suppressed.
However, as shown in FIG. 5, the two-dimensional hole gas layer (2DHG layer) 98 in the HEMT device 90 is formed around an interface between the electron transit layer 93 and the substrate 91 based on the hetero junction between the electron transit layer 93 and the substrate 91. The 2 DHG layer 98 functions as a current path between the source electrode and the drain electrode, but it is difficult that turn off control by a gate structure and the applied voltage. That is, the HEMT device 90 has a structure in which it is easy for the leak current to flow between the source electrode 95 and the drain electrode 96.
Further, since lattice mismatch (difference between lattice constants) is large between the substrate 91 made of AlN and the electron transit layer 93 made of GaN, there are easily generated many crystal defects, such as dislocations, in the interface between the substrate 91 and the electron transit layer 93. Consequently, the deterioration of electrical characteristic of the HEMT device 90, such as on-resistance or current collapse, occurs. Such a problem occurs similarly by use of a substrate made of AlxGa1-xN (0<x≦1).
This problem is not limited to the HEMT. The problem occurs similarly in lateral devices which include a substrate made of GaN and a hetero structure on the substrate, and in which electric current flows in a lateral direction (a direction parallel to a face of the substrate), when the device operates. Examples of such devices may be Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or Schottky Barrier Diode (SBD), etc.
Accordingly, it is difficult to manufacture the lateral device in which electrical characteristics are good on the substrate including AlN.